Polyethylene based plastic films currently enjoy a wide range of uses from simple product wrapping and packaging applications to high-tech applications such as the manufacture of high-altitude balloons useful in scientific and weather data collection applications. Plastic films are generally produced by an extrusion process in which a molten resin of polyethylene or other suitable plastic material is forced through a metal die having a predetermined shape and thickness. The extruded plastic shape is typically first expanded and then cooled to set the previously molten resin in the desired shape and thickness. In order to fabricate sheets of plastic film, the molten resin is generally extruded through a circular die to form a cylindrical film. The extruded cylindrical film is first substantially expanded and is then cooled, sliced along one or both of its longitudinal edges, and finally cut transversely to form sheets having the desired dimension.
The extrusion process used to form plastic film sheets is easily adaptable to produce films in a variety of sizes, thicknesses, colors and surface finishes. Furthermore, the tensile strength and elongation properties of the extruded film may be adjusted to the user's needs by modifying the blending and alloying process used to create the molten polyethylene resin. One of the most attractive qualities of extruded plastic films is that these films can provide an air-tight and water-tight protective barrier. Thus, plastic films offer a strong, light weight, inexpensive and easily producible alternative to other protective materials and fabrics.
Many applications of polyethylene based plastic films require the joining of multiple layers, sheets or sections of plastic film. In such applications, it is often desireable to join the sheet layers by forming one or more seals between individual film sheets. One particularly effective method for forming seals between sheets of plastic film involves the application of a predetermined amount of pressure and heat to a specific sheet area to melt the sheet layers together. This method is commonly referred to as heat sealing.
The heat sealing process, if performed properly, creates an air-tight and water-tight barrier between the sealed sheets with an integrity substantially equal to, if not greater than that of the plastic film sheets from which the seal is formed. A properly formed seal also mirrors the tensile and elongation properties of the constituent film sheets that form the seal. Thus, a plurality of individual sheets properly heat sealed together perform as well as an extruded single sheet of equivalent size.
Heat seals in plastic films are generally formed by a band sealer that, through the application of heat and pressure over a narrow area in the film sheets, partially melts and fuses the sheets of plastic film together. Movement of the plastic film sheets through the band sealer creates a seal of given length and width. Flaws and defects can be encountered during the sealing process when the band sealer temperature, transitional speed or pressure varies, when the film sheets wrinkle or twist during sealing, or when foreign material is introduced into the seal. The inclusion of flaws and defects into the seal produce weak seals that may deteriorate and fail under the intended specific application. Failure of the seal can have disastrous and costly effects ranging from product spoilage or loss in packaging applications to catastrophic failure in high-tech balloon applications.
There are several types of flaws and defects commonly encountered when sheets of polyethylene film are sealed by means of a band sealer. A "tuck" or "wrinkle" flaw generally occurs when a fold in the sheet material is located in the seal. A "smear" in the seal occurs when one edge of the seal is distorted or thinned out. A "debris" defect occurs when foreign matter, for example, loose polyethylene scraps, dust or dirt, is included in the seal. A "burn" defect occurs when the band sealer temperature is too high thereby causing the seal width to shrink or creating a melt-through hole in the seal. A "cloudy" seal occurs when the seal is hazy or appears not to be sealed through all sheet layers. A "dip" or "pocket" in the seal occurs when the seal width narrows or one edge of the seal is not straight. A "cold" seal easily pulls apart, and occurs when the band sealer temperature is not high enough to sufficiently melt the film layers and form a good seal.
Historically, heat seals formed between sheets of polyethylene plastic film have been visually and manually inspected by highly trained and experienced quality control inspectors who differentiate flawed or defective seals from good seals by examining the seal for various measurable properties such as clarity, thickness, width, straightness of edge, flatness and birefringent signature. "Clarity" refers to the amount of haziness in the seal. Flaws and defects are indicated by variations in clarity, both along the seal length and across the seal width. "Thickness" is the seal dimension measured normal to the surface plane of the constituent film sheets. Variations in seal thickness are generally caused by inconsistent application of heat and pressure, by inconsistent transitional movement of the film sheets through the band sealer or by inclusion of foreign matter into the seal. "Width" is the seal dimension in the surface plane of the film sheets. Again, inconsistences in the sealing process (heat, pressure, transition) will produce seals with width variations. "Straightness of edge" refers to whether the seal edge has a smooth, straight line or whether it is distorted or wavy in the surface plane of the film sheets along the seal length. "Flatness" refers to whether the seal will lay flat or becomes puckered or buckled when placed on a level surface. "Birefringent signature" refers to the pattern of colors exhibited by the seal area when viewed through a polariscope.
Almost all of the seal flaws and defects discussed above can be detected visually by examining the measurable properties. The exception is cold seals, which look virtually identical to good seals, but must be discovered manually by pulling on the seal. Effective and efficient quality control often requires multiple visual and manual inspections of the seal in order to guaranty its integrity. This complicated inspection process can only be accurately performed by trained and experienced inspectors and cannot be performed in real-time as the sealing process occurs. Therefore, the visual and manual inspection method is inconvenient, prohibitively costly and time consuming.
The visual and manual seal inspection and detection process is especially inconvenient, expensive and time consuming in the high-altitude balloon manufacturing industry. Inflatable balloons are generally comprised of a number of gores heat sealed together to form the balloon shape. These gores are structured of light weight, strong, non-porous, flexible extruded sheets of polyethylene film. The length of each heat seal formed between gore sheets may range anywhere from a few feet to several hundred feet long.
In common industry practice, balloons are manufactured by laying a plurality of balloon gores on a long flat table such that their common longitudinal edges are aligned. A band sealer, traveling along the common longitudinal edge to be sealed, then applies a specified amount of heat and pressure to melt and fuse adjacent gore members together. A load bearing tape (used to support the payload to be suspended under the balloon) and backup tape (used to support the seal) may also be placed along the longitudinal edge and incorporated into the seal. The sealing process is repeated for each additional gore needed to fabricate the completed balloon.
Flaws and defects are included in the heat seals formed between balloon gores when the band sealer temperature, pressure or transitional speed varies, when the film layers, load tapes or back-up tapes wrinkle, overlap, or twist, or when foreign material appears in the seal. Each of these flaws or defects may destroy the integrity of the seal rendering the fabricated balloon unusable. When possible, all detected flaws and defects are repaired. But an undetected flaw or defect could result in catastrophic balloon failure and complete loss of the balloon and its suspended payload upon inflation and launching. A finished balloon may incorporate hundreds of individual heat seals, each of which must be visually and manually inspected by the quality control inspector for flaws and defects before the balloon may be inflated. For an average size high-altitude balloon, the quality control inspectors may have to visually and manually inspect several miles of heat seals before qualifying the balloon for flight.
While reasonably efficient visual and manual inspection of heat seals can be performed by highly trained and experienced quality control personnel, this subjective approach places an enormous burden on the inspectors, provides no means of quality assurance, cannot be performed in real-time as the seals are formed and adds significantly to the balloon fabrication cost. Accordingly, there is a need for an apparatus that will reliably perform real-time inspection and examination of heat seals formed between sheets of plastic film (for example, balloon gores) to detect the presence of included flaws and defects.